Surface-treated steel plate for cell container

ABSTRACT

A surface-treated steel sheet for a battery container, including a steel sheet, an iron-nickel diffusion layer formed on the steel sheet, and a nickel layer formed on the iron-nickel diffusion layer (and constituting the outermost layer, wherein when the Fe intensity and the Ni intensity are continuously measured from the surface of the surface-treated steel sheet for a battery container along the depth direction with a high frequency glow discharge optical emission spectrometric analyzer, the thickness of the iron-nickel diffusion layer being the difference between the depth at which the Fe intensity exhibits a first predetermined value and the depth at which the Ni intensity exhibits a second predetermined value is 0.04 to 0.31 μm; and the total amount of the nickel contained in the iron-nickel diffusion layer and the nickel contained in the nickel layer is 4.4 g/m 2  or more and less than 10.8 g/m 2 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending application Ser. No.15/780,935 filed on Jul. 6, 2018, which is a U.S. National Stage ofInternational Application No. PCT/JP2016/086119 filed on Dec. 5, 2016,for which priority is claimed under 35 U.S.C. § 120; and thisapplication claims priority of Application No. 2015-236710 filed inJapan on Dec. 3, 2015 under 35 U.S.C. § 119; the entire contents of allof which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a surface-treated steel sheet for abattery container.

BACKGROUND ART

Recently, portable devices such as audio instruments and cellular phoneshave been used in various fields, and there have been used as theoperating power sources thereof many primary batteries such as alkalinebatteries and many secondary batteries such as nickel-hydrogen batteriesand lithium-ion batteries. Such batteries are demanded to achieve longoperating lives, high performances and the like by the achievement ofhigh performances of the devices being mounted with such batteries, andthe battery containers packed with power generation elements composed ofpositive electrode active materials, negative electrode active materialsand the like are also demanded to be improved in the performances as theimportant constituent elements of the batteries.

As the surface-treated steel sheets to form such battery containers, forexample, Patent Documents 1 and 2 disclose surface-treated steel sheetseach prepared by forming a nickel plating layer on a steel sheet, andthen forming an iron-nickel diffusion layer by applying a heat treatmentto the nickel plated steel sheet.

On the other hand, battery containers having a thin battery containerwall (hereinafter, referred to as “can wall”) have been demanded inorder to improve the volume percentage, under the requirements for theachievement of higher capacities and lighter weights of batteries. Forexample, as disclosed in Patent Documents 3 and 4, it has been known aprocessing allowing the thickness of the can wall after the processingto be thinner than the thickness of a surface-treated steel sheet beforethe processing.

RELATED ART Patent Documents

-   Patent Document 1: Japanese Patent Laid-Open No. 2014-009401-   Patent Document 2: Japanese Patent Laid-Open No. 6-2104-   Patent Document 3: International Publication No. WO 2009/107318-   Patent Document 4: International Publication No. WO 2014/156002

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, in Patent Documents 1 and 2, the heat treatment condition inthe formation of the iron-nickel diffusion layer is a high temperatureor a long time, and the inter-diffusion between the iron in the steelsheet serving as a substrate and the nickel in the nickel plating layertends to proceed in the resulting surface-treated steel sheet. Thepresent inventors have obtained a finding that when a heat treatment isperformed under the conventional heat treatment conditions, use of abattery with the surface treated steel sheet processed into a batterycontainer sometimes increases the amount of iron dissolved from theinner surface of the battery container, and the corrosion resistance isliable to be decreased. The iron exposed during formation of the batterycontainer is favorable for improving the battery properties; however astudy performed by the present inventors have revealed that whenthickness of the nickel plating layer foiled before heat treatment isthin, the exposure of the iron is locally increased. With increaseddissolution amount, the corrosion resistance is liable to be decreased.

In addition, in Patent Documents 3 and 4, there is a problem that byreducing the thickness of the can wall of the battery container, theamount of iron dissolved on the inner surface of the battery containersometimes comes to be increased, and the corrosion resistance of theinner surface of the battery container is decreased.

An object of the present invention is to provide a surface-treated steelsheet for a battery container excellent in corrosion resistance evenwhen the volume percentage is improved by reducing the thickness of thecan wall in the case where the surface-treated steel sheet is processedinto a battery container.

Means for Solving the Problem

According to the present invention, there is provided a surface-treatedsteel sheet for a battery container, including a steel sheet, aniron-nickel diffusion layer formed on the steel sheet, and a nickellayer formed on the iron-nickel diffusion layer and constituting theoutermost surface layer, wherein when the Fe intensity and the Niintensity are continuously measured along the depth direction from thesurface of the surface-treated steel sheet for a battery container, byusing a high frequency glow discharge optical emission spectrometricanalyzer, the thickness of the iron-nickel diffusion layer, being thedifference (D2−D1) between the depth (D1) at which the Fe intensityexhibits a first predetermined value and the depth (D2) at which the Niintensity exhibits a second predetermined value, is 0.04 to 0.31 μm, andthe total amount of nickel contained in the iron-nickel diffusion layerand the nickel layer is 4.4 g/m² or more and less than 10.8 g/m². It isto be noted that the depth (D1) exhibiting the first predetermined valueis the depth exhibiting an intensity of 10% of the saturated value ofthe Fe intensity measured by the above-described measurement, and thedepth (D2) exhibiting the second predetermined value is the depthexhibiting an intensity of 10% of the maximum value when the measurementis further performed along the depth direction after the Ni intensityshows the maximum value by the above-described measurement.

In the surface-treated steel sheet for a battery container of thepresent invention, the average crystal grain size in the surface portionof the nickel layer is preferably 0.2 to 0.6 μm.

In the surface-treated steel sheet for a battery container of thepresent invention, the thickness of the nickel layer is preferably 0.4to 1.2 μm.

In the surface-treated steel sheet for a battery container of thepresent invention, the Vickers hardness (HV) of the nickel layermeasured with a load of 10 gf is preferably 200 to 280.

According to the present invention, there is provided a batterycontainer made of the above-described surface-treated steel sheet for abattery container.

According to the present invention, there is also provided a batteryprovided with the above-described battery container.

Moreover, according to the present invention, there is provided a methodfor producing a surface-treated steel sheet for a battery container,including:

a nickel plating step of forming a nickel plating layer on a steel sheetwith a nickel amount of 4.4 g/m² or more and less than 10.8 g/m²; and

a heat treatment step of applying a heat treatment to the steel sheethaving the nickel plating layer formed thereon by maintaining the steelsheet at a temperature of 450 to 600° C. for 30 seconds to 2 minutes tothereby form an iron-nickel diffusion layer having a thickness of 0.04to 0.31 μm.

Effects of Invention

According to the present invention, it is possible to provide asurface-treated steel sheet for a battery container excellent incorrosion resistance even when the volume percentage is improved byreducing the thickness of the can wall when a battery container is madeof the surface-treated steel sheet for a battery container. Moreover,according to the present invention, it is possible to provide a batterycontainer and a battery obtained by using such a surface-treated steelsheet for a battery container.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an oblique perspective view showing one embodiment of abattery undergoing an application of the surface-treated steel sheet fora battery container according to the present invention.

FIG. 2 is a cross-sectional view along the line II-II in FIG. 1.

FIG. 3 is an enlarged cross-sectional view of the portion III in FIG. 2,in one embodiment of the surface-treated steel sheet for a batterycontainer of the present invention.

FIG. 4 is a diagram for illustrating the method for producing thesurface-treated steel sheet for a battery container shown in FIG. 3.

FIG. 5 presents the graphs showing the results of the Fe intensities andthe Ni intensities of the surface-treated steel sheets for a batterycontainer of Example and Comparative Examples, measured by a highfrequency glow discharge optical emission spectrometric analyzer.

FIG. 6 presents the graphs showing the results of the surface hardnessmeasurements of the surface-treated steel sheet for a battery containerof Example and Comparative Examples.

FIG. 7 presents the photographs showing the backscattered electronimages of the surface-treated steel sheets for a battery container ofExample and Comparative Examples.

FIG. 8 presents the photographs showing the backscattered electronimages, the element maps of iron and the element maps of nickel of theinner surface of the battery container formed of the surface-treatedsteel sheet for a battery container of Reference Example A.

FIG. 9 presents the photographs showing the backscattered electronimages, the element maps of iron and the element maps of nickel of theinner surface of the battery container formed of the surface-treatedsteel sheet for a battery container of Example 1.

FIG. 10 presents the photographs showing the backscattered electronimages, the element maps of iron and the element maps of nickel of theinner surface of the battery container formed of the surface-treatedsteel sheet for a battery container of Comparative Example 1.

FIG. 11 presents the photographs showing the backscattered electronimages, the element maps of iron and the element maps of nickel of theinner surface of the battery container formed of the surface-treatedsteel sheet for a battery container of Comparative Example 2.

FIG. 12 presents the photographs showing the backscattered electronimages of the surface-treated steel sheets for a battery container ofReference Examples.

FIG. 13 presents the photographs obtained by enlarging the backscatteredelectron images shown in the FIG. 12.

FIG. 14 is a graph showing the results of the measurements of theexposed area proportions of iron on the inner surfaces of the batterycontainers formed of the surface-treated steel sheets for a batterycontainer of Example and Comparative Example.

FIG. 15 is a graph showing the results of the measurement of the surfacehardness of a steel sheet having a nickel plating layer formed thereon,after the application of a heat treatment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, one Embodiment of the present invention is described by wayof the accompanying drawings. The surface-treated steel sheet for abattery container according to the present invention is processed intoan external shape corresponding to the desired shape of a battery.Examples of a battery may include, without being particularly limitedto: primary batteries such as an alkaline battery, and secondarybatteries such as a nickel-hydrogen battery and a lithium-ion battery;as the members of the battery containers of these batteries, thesurface-treated steel sheet for a battery container according to thepresent invention can be used. Hereinafter, the present invention isdescribed on the basis of an embodiment using the surface-treated steelsheet for a battery container according to the present invention for apositive electrode can constituting the battery container of an alkalinebattery.

FIG. 1 is an oblique perspective view showing one embodiment of analkaline battery 2 undergoing an application of the surface-treatedsteel sheet for a battery container according to the present invention,and FIG. 2 is a cross-sectional view along the line II-II in FIG. 1. Thealkaline battery 2 of the present example includes a positive electrodemixture 23 and a negative electrode mixture 24 filled inside thepositive electrode can 21 having a bottomed cylindrical shape throughthe intermediary of a separator 25; and a sealing body constituted witha negative electrode terminal 22, a current collector 26 and a gasket 27caulked on the inner surface side of the opening section of the positiveelectrode can 21. A convex positive electrode terminal 211 is formed inthe bottom center of the positive electrode can 21. In addition, anexterior case 29 is mounted on the positive electrode can 21 through theintermediary of an insulating ring 28, for the purpose of impartinginsulation properties, improving the design, and the like.

The positive electrode can 21 of the alkaline battery 2 shown in FIG. 1is obtained by mold-processing the surface-treated steel sheet for abattery container according to the present invention, by applying, forexample, a deep drawing processing method, a drawing and ironingprocessing method (DI processing method), a drawing thin and redrawingprocessing method (DTR processing method), or a processing method usinga stretch processing and an ironing processing after a drawingprocessing. Hereinafter, with reference to FIG. 3, the constitution ofthe surface-treated steel sheet for a battery container (surface-treatedsteel sheet 1) according to the present invention is described.

FIG. 3 is an enlarged cross-sectional view of the portion III of thepositive electrode can 21 shown in FIG. 2, and the upper side in FIG. 3corresponds to the inner surface (the surface in contact with thepositive electrode mixture 23 of the alkaline battery 2) of the alkalinebattery 2 of FIG. 1. The surface-treated steel sheet 1 of the presentembodiment includes, as shown in FIG. 3, an iron-nickel diffusion layer12 and a nickel layer 14 formed on a steel sheet 11 constituting thesubstrate of the surface-treated steel sheet 1.

In the surface-treated steel sheet 1 of the present embodiment, thethickness of the iron-nickel diffusion layer 12 measured by a highfrequency glow discharge optical emission spectrometric analyzer is 0.04to 0.31 μm, and the total amount of the nickel contained in theiron-nickel diffusion layer 12 and the nickel layer 14 is 4.4 g/m² ormore and less than 10.8 g/m². Herewith, the surface-treated steel sheet1 of the present embodiment is excellent in corrosion resistance even inthe case where the thickness of the can wall is reduced to improve thevolume percentage when the surface-treated steel sheet 1 is processedinto a battery container.

In addition, in the present embodiment, the average crystal grain sizein the surface portion of the nickel layer 14 is preferably 0.2 to 0.6μm. Herewith, the surface-treated steel sheet 1 of the presentembodiment comes to be more excellent in the corrosion resistance whenused as a battery container.

<Steel Sheet 11>

The steel sheet 11 of the present embodiment is not particularly limitedas long as the steel sheet 11 is excellent in molding processability;for example, a low carbon aluminum-killed steel (carbon content: 0.01 to0.15% by weight), a low carbon steel having a carbon content of 0.003%by weight or less, or a non-aging low carbon steel prepared by addingTi, Nb or the like to a low carbon steel can be used. The thickness ofthe steel sheet is not particularly limited, but is preferably 0.2 to0.5 mm. When the steel sheet is too thick, the heat quantity necessaryfor diffusion is deficient, and the diffusion layer is liable to beformed insufficiently. When the steel sheet is too thin, the thicknessnecessary as the subsequent battery sometimes cannot be secured, or theheat conduction is fast and the control of the thickness of thediffusion layer is liable to be difficult.

In the present embodiment, the hot rolled sheets of these steels arewashed with an acid to remove the scales (oxide film), then cold rolled,then electrolytically washed, then annealed and subjected to temperrolling, and are used as the steel sheets 11; or alternatively, the hotrolled sheets of these steels washed with an acid to remove the scales(oxide film), then cold rolled, then electrolytically washed, thensubjected to temper rolling without being subjected to annealing, andare used as the steel sheets 11.

<Iron-Nickel Diffusion Layer 12, and Nickel Layer 14>

In the surface-treated steel sheet 1 of the present embodiment, theiron-nickel diffusion layer 12 is a layer allowing iron and nickel tomutually diffuse therein, formed as a result of a thermal diffusiontreatment performed after the nickel plating layer 13 is formed on thesteel sheet 11, so as to cause the thermal diffusion of the ironconstituting the steel sheet 11 and the nickel constituting the nickelplating layer 13. The nickel layer 14 is a layer derived from theportion free from the diffusion of iron in the vicinity of the surfacelayer of the nickel plating layer 13, the portion being thermallyrecrystallized and softened when the thermal diffusion treatment isperformed.

By forming the iron-nickel diffusion layer 12 obtained by such a thermaldiffusion treatment, when the surface-treated steel sheet 1 is used as abattery container, the direct, wide area contact of the steel sheet withthe electrolytic solution or the like constituting the battery can beprevented; and moreover, the presence of the iron-nickel diffusion layer12 relaxing the potential difference between the nickel of the nickellayer 14 and the iron of the steel sheet 11 allows the corrosionresistance and the battery properties to be satisfactory. The formationof the iron-nickel diffusion layer 12 also allows the adhesivenessbetween the steel sheet 11 and the nickel layer 14 to be improved.

The nickel plating layer 13 for forming the iron-nickel diffusion layer12 can be formed on the steel sheet 11 by using, for example, a nickelplating bath. As the nickel plating bath, there can be used a platingbath usually used in nickel plating, namely, a Watts bath, a sulfamatebath, a boron fluoride bath, a chloride bath and the like. For example,the nickel plating layer 13 can be formed by using, as a watts bath, abath having a bath composition containing nickel sulfate in aconcentration of 200 to 350 g/L, nickel chloride in a concentration of20 to 60 g/L, and boric acid in a concentration of 10 to 50 g/L, underthe conditions that the pH is 3.0 to 4.8 (preferably pH is 3.6 to 4.6),the bath temperature is 50 to 70° C., the current density is 10 to 40A/dm² (preferably 20 to 30 A/dm²).

It is to be noted that as the nickel plating layer, a sulfur-containingbright plating is not preferable because the battery properties areliable to be degraded; however, it is possible in the present inventionto apply a matte plating not containing sulfur in an amount equal to ormore than the amount of an inevitable impurity as well as a semi-glossplating. This is because the hardness of the layer obtained by platingis as follows: the semi-gloss plating is harder than the matte plating,but the heat treatment for forming the diffusion layer in the presentinvention allows the hardness of the semi-gloss plating to be comparablewith or slightly higher than the hardness of the matte plating. When asemi-gloss plating is formed as a nickel plating layer, a semi-glossagent may be added to the above-described plating baths. The semi-glossagent is not particularly limited as long as the semi-gloss agent allowsthe nickel plating layer after plating to be free from sulfur (forexample, 0.05% or less in an fluorescent X-ray measurement); as thesemi-glass agent, it is possible to use, for example, an aliphaticunsaturated alcohol such as a polyoxyethylene adduct of an unsaturatedalcohol, an unsaturated carboxylic acid, formaldehyde, and coumarin.

In the present embodiment, as shown in FIG. 4, the above-describednickel plating layer 13 is for ed on the steel sheet 11, andsubsequently a thermal diffusion treatment is performed; thus, theiron-nickel diffusion layer 12 and the nickel layer 14 are formed, andthe surface-treated steel sheet 1 as shown in FIG. 3 can be obtained.

In the present embodiment, the nickel amount in the nickel plating layer13 before performing the thermal diffusion treatment corresponds to thetotal amount of the nickel contained in the iron-nickel diffusion layer12 and the nickel contained in the nickel layer 14 obtained by thethermal diffusion treatment.

The total amount (the nickel amount in the nickel plating layer 13before performing the thermal diffusion treatment) of the nickelcontained in the iron-nickel diffusion layer 12 and the nickel containedin the nickel layer 14 obtained by the thermal diffusion treatment maybe 4.4 g/m² or more and less than 10.8 g/m², is preferably 5.5 g/m² ormore and less than 10.8 g/m², and is more preferably 6.5 g/m² or moreand less than 10.8 g/m². When the total amount of the nickel containedin the iron-nickel diffusion layer 12 and the nickel contained in thenickel layer 14 is too small, the improvement effect of the corrosionresistance due to nickel is insufficient, and the corrosion resistanceof the obtained surface-treated steel sheet 1 used as a batterycontainer is degraded. On the other hand, when the total amount of thenickel contained in the iron-nickel diffusion layer 12 and the nickelcontained in the nickel layer 14 is too large, the can wall thickness ofthe battery container made of the obtained surface-treated steel sheet 1comes to be thick and the volume of the interior of the batterycontainer comes to be small (the volume percentage is degraded). Thetotal amount of the nickel contained in the iron-nickel diffusion layer12 and the nickel contained in the nickel layer 14 can be determined bya method calculating on the basis of the total amount (total weight) ofthe nickel contained in the iron-nickel diffusion layer 12 and thenickel contained in the nickel layer 14 measurable with an ICP analysismethod. Alternatively, the total amount of the nickel contained in theiron-nickel diffusion layer 12 and the nickel contained in the nickellayer 14, can also be determined by a method calculating on the basis ofthe measured deposition amount obtained by measuring the depositionamount of the nickel atoms constituting the nickel plating layer 13 byperforming a fluorescent X-ray measurement after the formation of thenickel plating layer 13 and before performing the thermal diffusiontreatment.

The conditions of the thermal diffusion treatment may be appropriatelyselected according to the thickness of the nickel plating layer 13; theheat treatment temperature is 450 to 600° C., more preferably 480 to590° C., and further preferably 500 to 550° C.; the soaking time in theheat treatment is preferably 30 seconds to 2 minutes, more preferably 30to 100 seconds, and further preferably 45 to 90 seconds. In addition, inthe heat treatment, the time including the heating up time and thecooling time in addition to the soaking time is preferably 2 to 7minutes and more preferably 3 to 5 minutes. The thermal diffusiontreatment method is preferably a continuous annealing method from theviewpoint of easy regulation of the heat treatment temperature and theheat treatment time within the above-described ranges.

In the present invention, as described above, by performing the thermaldiffusion treatment, the iron-nickel diffusion layer 12 can be formedbetween the steel sheet 11 and the nickel layer 14, and consequently thesurface-treated steel sheet 1 is allowed to have a constitution(Ni/Fe-Ni/Fe) having the iron-nickel diffusion layer 12 and the nickellayer 14 on the steel sheet 11 in order from bottom to top.

In the present embodiment, the thickness of the thus formed iron-nickeldiffusion layer 12 measured with a high frequency glow discharge opticalemission spectrometric analyzer may be 0.04 to 0.31 μm, and ispreferably 0.05 to 0.27 μm, more preferably 0.08 to 0.25 μm, and furtherpreferably 0.09 to 0.20 μm. When the thickness of the iron-nickeldiffusion layer 12 is too small, the adhesiveness of the nickel layer 14in the surface-treated steel sheet 1 is liable to be degraded. On theother hand, when the thickness of the iron-nickel diffusion layer 12 istoo large, the amount of exposed iron comes to be too large in thenickel layer 14 of the surface-treated steel sheet 1, and consequently,when the surface-treated steel sheet 1 is used as the battery container,the amount of iron dissolved from the inner surface of the batterycontainer is large and the corrosion resistance is degraded.

It is to be noted that the thickness of the iron-nickel diffusion layer12 can be determined by continuously measuring the variations of the Feintensity and the Ni intensity in the depth direction from the outermostsurface toward the steel sheet 11 with respect to the surface-treatedsteel sheet 1 by using a high frequency glow discharge optical emissionspectrometric analyzer.

Specifically, first, by using the high frequency glow discharge opticalemission spectrometric analyzer, the Fe intensity in the surface-treatedsteel sheet 1 is measured until the Fe intensity is saturated, and byadopting the saturated value of the Fe intensity as a reference, thedepth giving the Fe intensity 10% of the saturated value is defined asthe boundary between the nickel layer 14 and the iron-nickel diffusionlayer 12. For example, the details are described with reference to FIG.5(A) showing the measurement results of the surface-treated steel sheet1 of a below-described Example. It is to be noted that, in FIG. 5(A),the ordinate represents the Fe intensity and the Ni intensity, and theabscissa represents the measurement time when the measurement isperformed in the depth direction from the surface of the surface-treatedsteel sheet 1 by using a high frequency glow discharge optical emissionspectrometric analyzer.

In the present embodiment, first, the saturated value of the Feintensity is determined on the basis of the measurement results of theFe intensity. The saturated value of the Fe intensity is determined fromthe time variation rate (Fe intensity variation/second) of the Feintensity. The time variation rate of the Fe intensity comes to besteeply large when Fe is detected after the start of the measurement,and decreases after passing the maximum value and is stabilized in thevicinity of approximately zero. The value when the time variation rateis stabilized at approximately zero is the saturated value, andspecifically, when the time variation rate of the Fe intensity comes tobe 0.02 (Fe intensity variation/second) or less, the measurement time inthe depth direction can be taken as the measurement time of thesaturation of the Fe intensity.

In the example shown in FIG. 5(A), the saturated value of the Feintensity is a value of approximately 70 in the vicinity of themeasurement time of 20 seconds, and the depth giving an Fe intensity ofapproximately 7, 10% of the saturated value, can be detected as theboundary between the nickel layer 14 and the iron-nickel diffusion layer12.

On the other hand, the boundary between the iron-nickel diffusion layer12 and the steel sheet 11 can be detected as follows. Specifically, whenthe Ni intensity of the surface-treated steel sheet 1 is measured byusing a high frequency glow discharge optical emission spectrometricanalyzer, the maximum value is extracted from the obtained graph showingthe variation of the Ni intensity, and the depth giving a Ni intensityof 10% of the maximum value after the maximum value has been shown isdetermined as the boundary between the iron-nickel diffusion layer 12and the steel sheet 11. For example, with reference to FIG. 5(A), themaximum value of the Ni intensity is a value of approximately 70 at themeasurement time in the vicinity of 9 seconds, and accordingly, thedepth giving a Ni intensity of approximately 7, 10% of the maximum valueof the Ni intensity, can be detected as the boundary between the nickelplating layer 13 and the steel sheet 11.

In addition, in the present embodiment, on the basis of the boundariesbetween the layers determined as described above, it is possible todetermine the thickness of the iron-nickel diffusion layer 12.Specifically, when the measurement is performed by using a highfrequency glow discharge optical emission spectrometric analyzer, thetime giving an Fe intensity of 10% of the saturated value of the Feintensity is set as the starting point, the measurement time until thetime giving a Ni intensity of 10% of the maximum value after the Niintensity has exhibited the maximum value is calculated, and on thebasis of the calculated measurement time, the thickness of theiron-nickel diffusion layer 12 can be determined.

It is to be noted that for the purpose of determining the thickness ofthe iron-nickel diffusion layer 12 of the surface-treated steel sheet 1on the basis of the measurement time, the high frequency glow dischargeoptical emission spectrometric analysis of the nickel-plated steel sheethaving a known thickness and having undergone no thermal diffusiontreatment is performed, the depth thickness calculated as theiron-nickel diffusion layer seen in the measurement graph (for example,FIG. 5(C) showing the measurement results of below-described ComparativeExample 1) is required to be subtracted at the time of calculation ofthe iron-nickel diffusion layer 12 of the surface-treated steel sheet 1as the actual measurement object. Specifically, from the thickness ofthe iron-nickel diffusion layer 12 portion (the thickness value obtainedin FIG. 5(A) as follows: the time giving the Fe intensity of 10% of thesaturated value of the Fe intensity is taken as the starting point, themeasurement time until the time giving the Ni intensity an intensity of10% of the maximum value of the Ni intensity after the Ni intensity hasexhibited the maximum value thereof is converted into the thickness)calculated from the graph of FIG. 5(A), the thickness calculated in thesame manner from the graph of FIG. 5(B) is subtracted, and thus, thethickness of the actual iron-nickel diffusion layer 12 in the graph ofFIG. 5(A) can be determined.

In the present invention, as described above, for the nickel-platedsteel sheet having a known plating thickness and having undergone noheat treatment, a high frequency glow discharge optical emissionspectrometric analysis is performed, the thickness calculated as aniron-nickel diffusion layer is taken as the “reference thickness,” andthe difference (D2−D1) between D1 and D2 indicates the value obtained bysubtracting the reference thickness.

It is to be noted that in the measurement with a high frequency glowdischarge optical emission spectrometric analyzer, with the increase ofthe thickness of the nickel plating layer, the reference thicknesscalculated from the measurement of the nickel plating layer comes to beincreased; thus, when the thickness of the iron-nickel diffusion layeris determined, the reference thickness is checked in the platingdeposition amount of each of the layers, or alternatively, it isdesirable that the measurement of the reference thickness is performedin each of the two or more samples, different from each other in theplating deposition amount before performing heat treatment, the relationformula between the plating deposition amount and the referencethickness is determined, and then the thickness of the iron-nickeldiffusion layer is calculated.

In addition, by measuring the nickel-plated steel sheet undergoing nothermal diffusion treatment, the relation between the depth time (themeasurement time based on a high frequency glow discharge opticalemission spectrometric analyzer) and the actual thickness can bedetermined, and accordingly, by utilizing this numerical value (thenumerical value showing the relation between the depth time and theactual thickness), it is possible to convert the depth times into thethickness of the iron-nickel diffusion layer 12 and the thickness of thenickel layer 14 of the surface-treated steel sheet 1, to be an actualmeasurement object.

It is to be noted that when the thickness of the iron-nickel diffusionlayer 12 is measured as described above with a high frequency glowdischarge optical emission spectrometric analyzer, sometimes there is adetection limit value of the thickness of the iron-nickel diffusionlayer 12, due to the performances of the high frequency glow dischargeoptical emission spectrometric analyzer, the measurement conditions orthe like. For example, when a heat-treated nickel-plated steel sheet 1prepared by using, as the steel sheet 11, a steel sheet having a surfaceroughness Ra of 0.05 to 3 μm, as measured with a stylus-type roughnessmeter, is measured with a measurement diameter of ϕ5 mm of a highfrequency glow discharge optical emission spectrometric analyzer, thedetectable region (detection limit value with respect to shape) isapproximately 0.04 μm; when the thickness of the iron-nickel diffusionlayer 12 measured with the high frequency glow discharge opticalemission spectrometric analyzer is equal to or less than the detectionlimit value, the thickness of the iron-nickel diffusion layer 12 can beregarded to be more than 0 μm and less than 0.04 μm. In other words, inthe case where the nickel plating layer 13 is formed on the steel sheet11, and subsequently the iron-nickel diffusion layer 12 and the nickellayer 14 are formed by performing a thermal diffusion treatment, evenwhen the thickness of the iron-nickel diffusion layer 12 is equal to orless than the detection limit value in the measurement of the thicknessof the iron-nickel diffusion layer 12 by using the high frequency glowdischarge optical emission spectrometric analyzer, the thickness of theiron-nickel diffusion layer 12 can be regarded to be more than 0 μm andless than 0.04 μm. It is to be noted that when the nickel plating layer13 is formed on the steel sheet 11, and then a nickel-plated steel sheetis obtained by applying no thermal diffusion treatment, the iron-nickeldiffusion layer 12 can be regarded not to be formed in the nickel-platedsteel sheet (the thickness of the iron-nickel diffusion layer 12 is 0).

The thickness of the iron-nickel diffusion layer 12 is increased withthe increase of the heat treatment temperature, or with the increase ofthe heat treatment time which allows the mutual diffusion of iron andnickel to proceed easily. Because iron and nickel mutually diffuse, theformed iron-nickel diffusion layer 12 extends on the side of the steelsheet 11 and also diffuses on the side of the nickel plating layer 13,in relation to the interface between the steel sheet 11 and the nickelplating layer 13 before the diffusion. When the heat treatmenttemperature is set to be too high, or the heat treatment time is set tobe too long, the iron-nickel diffusion layer 12 comes to be thick, andthe nickel layer 14 comes to be thin. For example, the thickness of theiron-nickel diffusion layer 12 comes to be more than 0.3 μm. The presentinventors have discovered that when such a surface-treated steel sheetis molded into a battery container, there occurs an increase of thedissolution amount probably caused by the increase of the exposure ofiron. The causes for the exposure of iron on the inner surface of thebattery container are probably the exposure of a large amount of iron onthe inner surface of the battery container and the appearance of localiron exposure portions, not only in the case where the thickness of thenickel layer 14 nearly vanishes and iron reaches the surface layer inthe surface-treated steel sheet 1, but also in the case where iron doesnot reach the surface layer in the state of the surface-treated steelsheet 1. In this case, when the surface-treated steel sheet 1 is storedor used as a battery container over a long term, there is an adversepossibility that iron is dissolved from the local iron exposure portionsinto the electrolytic solution, and the gas generated due to thedissolution of iron increases the internal pressure of the interior ofthe battery.

In particular, the present inventors have discovered that the corrosionresistance is liable to be more degraded, in the case where the nickelplating layer is made thin for the purpose of achieving a high batterycapacity, or in the case where a processing is performed to make thethickness of the can wall after forming a battery can thinner than thethickness of the surface-treated steel sheet before forming the batterycan; the present inventors have revealed that the surface-treated steelsheet 1 of the present embodiment exhibits a marked corrosion resistanceeven under such severe processing conditions. Moreover, for the purposeof achieving a high battery capacity, it is possible to make thin thethickness of the nickel plating layer and to make thin the thickness ofthe can wall; however, either of these approaches offers a factor todegrade the corrosion resistance of the battery container. The presentinventors have found a new problem of the compatibility of theseapproaches for achieving a high capacity and the corrosion resistanceimprovement with respect to the conventional surface-treated steelsheets, and have found a new constitution capable of coping with theachievement of a high capacity.

In the present embodiment, as described above, with respect to thesurface-treated steel sheet 1, by controlling the total amount of thenickel contained in the iron-nickel diffusion layer and the nickelcontained in the nickel layer so as to fall within a comparatively smallrange of 4.4 g/m² or more and less than 10.8 g/m², the thickness of thecan wall can be made thin when the surface-treated steel sheet 1 isformed into a battery container, and accordingly the volume percentageof the obtained battery can be remarkably improved. Moreover, accordingto the surface-treated steel sheet 1 of the present embodiment, bysetting the thickness of the iron-nickel diffusion layer 12 to be 0.04to 0.31 μm, even when the thickness of the can wall is made thin toimprove the volume percentage as described above in the case where thesurface-treated steel sheet 1 is formed into a battery container, thebattery container can be made excellent in corrosion resistance. It isto be noted that when thickness of the can wall of a battery containeris made thin, the amount of iron dissolved on the inner surface of thebattery container sometimes has hitherto come to be large, andconsequently the corrosion resistance of the inner surface of thebattery container is sometimes degraded. On the other hand, as a methodfor improving the corrosion resistance when formed into a batterycontainer, there is a method to make thick the thickness of theiron-nickel diffusion layer and the thickness of the nickel layer formedon the inner surface of the battery container; however, in this case,there is a problem that the thickness of the can wall comes to be thickwhen formed into a battery container, and consequently the volumepercentage is degraded. Accordingly, in the technique for thesurface-treated steel sheet for a battery container, it has beendifficult to allow the volume percentage and the corrosion resistance tobe compatible with each other when formed into a battery container. Incontrast, according to the present embodiment, by controlling thethickness of the iron-nickel diffusion layer 12 and the above-describedtotal amount of the nickel contained in the iron-nickel diffusion layer12 and the nickel contained in the nickel layer 14 so as to fall withinthe above-described ranges, respectively, it is possible to provide asurface-treated steel sheet 1 being highly balanced between the volumepercentage and the corrosion resistance when formed into a batterycontainer.

In addition, there has hitherto been known a method in which thethickness of the iron-nickel diffusion layer is set to be 0.5 μm ormore, in the surface-treated steel sheet having a nickel plating layerand an iron-nickel diffusion layer, for example, from the viewpoint ofimproving the processability when molded as a battery container, fromthe viewpoint of improving the corrosion resistance of the batterycontainer, and from the viewpoint of securing the adhesiveness of theiron-nickel diffusion layer (see, for example, the paragraph 0018 inJapanese Patent Laid-Open No. 2009-263727). Herein, in order to set thethickness of the iron-nickel diffusion layer to be 0.5 μm or more, thecondition of the thermal diffusion treatment after the formation of thenickel plating layer on the steel sheet is required to be a long time ora high temperature. For example, when the condition of the thermaldiffusion treatment is set to be a long time, there have been known theconditions that the heat treatment temperature is set to be 400 to 600°C., and the heat treatment time is set to be 1 to 8 hours. In addition,when the condition of the thermal diffusion treatment is set to be ahigh temperature, there have been known the conditions that the heattreatment temperature is set to be 700 to 800° C., and the heattreatment time is set to be 30 seconds to 2 minutes. Under suchcircumstances, the present inventors have obtained a finding that whenthe thermal diffusion treatment is performed under the above-describedcondition of a long time or a high temperature, the iron of the steelsheet constituting the surface-treated steel sheet is thermally diffusedto an excessive extent, and when the obtained surface-treated steelsheet is molded into a battery container, the amount of iron dissolvedis increased; and accordingly, as described above, the present inventorshave discovered that gas is generated in the interior of the battery,and the internal pressure of the interior of the battery is liable to beincreased due to the generation of the gas. In addition, when thethermal diffusion treatment is performed at a heat treatment temperatureof 700 to 800° C. and at a heat treatment time of 30 seconds to 2minutes, there is a problem that the hardness of the nickel layer 14 isdecreased excessively, and consequently the sticking to the mold occursto a large extent.

In contrast, according to the present embodiment, with respect to thesurface-treated steel sheet 1, by setting the thickness of theiron-nickel diffusion layer 12 to be 0.04 to 0.31 μm, and by controllingthe total amount of the nickel contained in the iron-nickel diffusionlayer and the nickel contained in the nickel layer so as to fall withina range of 4.4 g/m² or more and less than 10.8 g/m², the exposure areaof the iron of the steel sheet is reduced on the inner surface side whenthe steel sheet surface-treated steel sheet 1 is molded into a batterycontainer, it is made possible to improve the corrosion resistance whenthe surface-treated steel sheet 1 is used as a battery container, and inaddition, it is made possible to more improve the processability whenthe surface-treated steel sheet 1 is processed into a battery container.

In addition, in the present embodiment, the thickness of the nickellayer 14 after the thermal diffusion treatment is preferably 0.5 to 1.20μm, more preferably 0.60 to 1.20 μm, and further preferably 0.70 to 1.17μm. By controlling the thickness of the nickel layer 14 after thethermal diffusion treatment so as to fall within such a comparativelythin range as described above, while the corrosion resistanceimprovement effect due to the iron-nickel diffusion layer 12 is beingsufficiently secured, it is possible to make thin the wall thicknesswhen molded into a battery container, and thus, it is possible toincrease the volume inside the battery container. Herewith, the amountsof the contents such as the positive electrode mixture 23 and thenegative electrode mixture 24 to be packed in the battery container canbe increased, and the battery properties of the obtained battery can beimproved. The thickness of the nickel layer 14 after the thermaldiffusion treatment can be determined by detecting the boundary betweenthe nickel layer 14 and the iron-nickel diffusion layer 12, on the basisof the measurement using the above-described high frequency glowdischarge optical emission spectrometric analyzer. In other words, thetime at which the measurement of the surface of the surface-treatedsteel sheet 1 is started by using the high frequency glow dischargeoptical emission spectrometric analyzer is taken as the starting point,the measurement time until the time giving the Fe intensity of 10% ofthe saturated value of the Fe intensity is calculated, and on the basisof the calculated measurement time, the thickness of the nickel layer 14can be determined.

In addition, in the present embodiment, in the nickel layer 14 after thethermal diffusion treatment, the average crystal grain size in thesurface portion thereof is preferably 0.2 to 0.6 μm, more preferably 0.3to 0.6 μm, and further preferably 0.3 to 0.5 μm. In the presentembodiment, the average crystal grain size in the surface portion of thenickel layer 14 is not particularly limited; when the average crystalgrain size is too small, the plating stress remains accumulated, and inthis case, when mold-processed as a battery container, a deep crackreaching the steel sheet occurs in the surface-treated steel sheet 1,and thus, the iron of the steel sheet 11 is sometimes exposed. In thiscase, iron is dissolved from the exposed portion of the steel sheet 11,and there is an adverse possibility that the gas generated along withthe dissolution of iron increases the internal pressure of the interiorof the battery. On the other hand, as described above, failures occurwhen the cracks reaching the steel sheet 11 are generated in thesurface-treated steel sheet 1; however, from the viewpoint of improvingthe battery properties of the battery container, it is preferable forfine cracks to occur on the inner surface side of the battery containerformed of the surface-treated steel sheet 1. In this regard, when theaverage crystal grain size in the surface portion of the nickel layer 14is too large, the hardness of the nickel layer 14 sometimes comes to betoo low (the nickel layer 14 is softened excessively); in this case,when the surface-treated steel sheet 1 is mold-processed as a batterycontainer, fine cracks cannot be generated on the inner surface of thebattery container, and accordingly, there is an adverse possibility thatthe following effect is not sufficiently obtained: the effect ofimproving the battery properties, namely, the effect of improving thebattery properties by increasing the contact area between the batterycontainer and the positive electrode mixture owing to the cracks andthereby decreasing the internal resistance of the battery.

According to the present embodiment, with respect to the surface-treatedsteel sheet 1, even in the case where the thickness of the nickelplating layer 13 is set to be comparatively thin, by setting thethickness of the iron-nickel diffusion layer 12 to be comparatively asthin as 0.04 to 0.31 μm, the exposure area of the iron of the steelsheet is suppressed on the inner surface side when the steel sheetsurface-treated steel sheet 1 is molded into a battery container, andthus, it is made possible to improve the corrosion resistance when thesurface-treated steel sheet 1 is used as a battery container. Inaddition, according to the present embodiment, by controlling theaverage crystal grain size in the surface portion of the nickel layer 14so as to be 0.2 to 0.6 μm, it is made possible to more improve theprocessability when the surface-treated steel sheet 1 is processed intoa battery container. Moreover, according to the present embodiment, thethickness of the iron-nickel diffusion layer 12 and the thickness of thenickel layer 14 are set to be comparatively thin, accordingly, it isadvantageous in terms of cost to produce the surface-treated steel sheet1, it is made possible to increase the internal volume of the batterycontainer when the surface-treated steel sheet 1 is molded into abattery container and a battery is assembled, and thus the improvementof the battery properties is resulted.

It is to be noted that the average crystal grain size in the surfaceportion of the nickel layer 14 tends to be larger with the increase ofthe heat treatment temperature in the thermal diffusion treatment, andthe present inventors have discovered that the magnitude of the averagecrystal grain size increases in a stepwise manner depending on thetemperature range. The crystal grains are larger in the case where heattreatment is applied even at a low temperature such as 300° C., ascompared with the case where no heat treatment is applied. When the heattreatment temperature is set to be between 400 and 600° C., the crystalgrain size increases with the increase of the temperature, but thedifference of the magnitude of the crystal grain size due to thetemperature is moderate. When the heat treatment temperature exceeds700° C., the average crystal grain size steeply increases. Accordingly,by controlling the heat treatment temperature of the thermal diffusiontreatment, it is possible to regulate the average crystal grain size inthe surface portion of the nickel layer 14. In particular, bysuppressing the coarsening of the average crystal grain size andallowing the surface hardness of the nickel layer 14 to be hard, it ismade possible to aim at the improvement of the battery properties andthe suppression effect of the sticking of the nickel layer 14 to themold during the processing into the battery container, and accordinglythe heat treatment temperature is particularly preferably 430 to 550° C.In other words, by allowing the surface hardness of the nickel layer 14to be hard by setting the heat treatment temperature so as to fallwithin the above described range, it is made possible to generate finecracks not reaching the steel sheet 11, on the inner surface of thebattery container made of the surface-treated steel sheet 1 when thesurface-treated steel sheet 1 is mold-processed into a batterycontainer, the cracks increases the contact area between the batterycontainer and the positive electrode mixture and decreases the internalresistance of the battery, and thus the battery properties can befurther improved.

In the present embodiment, the average crystal grain size in the surfaceportion of the nickel layer 14 can be determined, for example, by usingthe backscattered electron image obtained by measuring the surface ofthe surface-treated steel sheet 1 with a scanning electron microscope(SEM).

Specifically, first, the surface of the surface-treated steel sheet 1 isetched if necessary, then the surface of the surface-treated steel sheet1 is measured with a scanning electron microscope (SEM), as shown inFIG. 7W. It is to be noted that FIG. 7(A) is an image showing thebackscattered electron image obtained by measuring the surface-treatedsteel sheet 1 of below-described Example, at an magnification of 10,000.Then, on the obtained backscattered electron image, an optional numberof straight line segments of 10 μm in length are drawn (four lines, forexample). Then, in each of the line segments, on the basis of the numbern of the crystal grains located on the straight line segment, thecrystal grain size d is determined by using the formula d=10/(n+1), andthe average value of the crystal grain sizes d obtained for therespective straight line segments can be taken as the average crystalgrain size in the surface portion of the nickel plating layer 13.

In addition, in the present embodiment, the surface hardness of thenickel layer 14 after the thermal diffusion treatment is preferably 200to 280, and more preferably 210 to 250, in terms of the Vickers hardness(HV) measured with a load of 10 gf. By setting the surface hardness ofthe nickel layer 14 after the thermal diffusion treatment so as to fallwithin the above-described range, the processability is improved whenthe obtained surface-treated steel sheet 1 is processed into a batterycontainer, and the corrosion resistance is improved when thesurface-treated steel sheet 1 is used for the battery container.

In the present embodiment, with respect to the surface-treated steelsheet 1, as a method for controlling the thickness of the iron-nickeldiffusion layer 12 and the total amount of the nickel contained arecontrolled in the iron-nickel diffusion layer and the nickel containedin the nickel layer so as to fall within the above-described ranges,respectively, a method for performing the thermal diffusion treatmentunder the above-described conditions may be mentioned. Specifically,there may be mentioned a method in which after the nickel plating layer13 is formed on the steel sheet 11, a thermal diffusion treatment isperformed under the conditions that the heat treatment temperature is450 to 600° C., and the heat treatment time is 30 seconds to 2 minutes.

In addition, in the present embodiment, with respect to the obtainedsurface-treated steel sheet 1, also as the method for controlling theaverage crystal grain size in the surface portion of the nickel layer 14so as to fall within the above-described range, a method of performing athermal diffusion treatment under the same conditions as described abovemay be mentioned. Specifically, there may be mentioned a method in whichafter the nickel plating layer 13 is formed on the steel sheet 11, athermal diffusion treatment is pertained under the conditions that theheat treatment temperature is 450 to 600° C., and the heat treatmenttime is 30 seconds to 2 minutes.

It is to be noted that the thickness of the iron-nickel diffusion layer12 tends to be thick, with the increase of the heat treatmenttemperature, and with the increase of the heat treatment time.Accordingly, by controlling the heat treatment temperature and the heattreatment time of the thermal diffusion treatment, it is possible toregulate the thickness of the iron-nickel diffusion layer 12 and theratio of (thickness of iron-nickel diffusion layer 12/thickness ofnickel layer 14). However, because it is difficult to form aniron-nickel diffusion layer at 300° C., from the viewpoint ofcontrolling the thickness of the iron-nickel diffusion layer 12, and theratio of (thickness of iron-nickel diffusion layer 12/thickness ofnickel layer 14) so as to fall within the above-described ranges, it ispreferable to perform a thermal diffusion treatment at 480° C. orhigher.

The surface-treated steel sheet 1 of the present embodiment isconstituted as described above.

The surface-treated steel sheet 1 of the present embodiment is used asmold-processed into the positive electrode can 21 of an alkaline battery2 shown in FIGS. 1 and 2, battery containers of other batteries and thelike, by using, for example, a deep drawing processing method, a drawingand ironing processing method (DI processing method), a drawing thin andredrawing processing method (DTR processing method), or a processingmethod using a stretch processing and an ironing processing incombination after a drawing processing.

<Method for Producing Surface-Treated Steel Sheet 1>

Next, a method for producing the surface-treated steel sheet 1 of thepresent embodiment is described.

First, the steel sheet 11 is prepared, and as described above, a nickelplating is applied to the steel sheet 11, to form the nickel platinglayer 13 on the surface of the steel sheet 11, to be the inner surfaceof a battery container. It is to be noted that the nickel plating layer13 is preferably formed not only on the surface of the steel sheet 11 tobe the inner surface of the battery container but also on the oppositesurface. When the nickel plating layer 13 is formed on both surfaces ofthe steel sheet 11, the nickel plating layers 13 different from eachother in the composition and the surface roughness may be formed on thesurface in the steel sheet 11 to be the inner surface of the batterycontainer and on the surface of the steel sheet 11 to be the outersurface of the battery container, respectively, by using plating bathshaving different compositions; however, from the viewpoint of improvingthe production efficiency, it is preferable to form the nickel platinglayers 13 on both surfaces of the steel sheet 11, by using the sameplating bath in one step.

Next, by performing the thermal diffusion treatment under theabove-described conditions for the steel sheet 11 having the nickelplating layer 13 formed thereon, the iron constituting the steel sheet11 and the nickel constituting the nickel plating layer 13 are allowedto thermally diffuse, to from the iron-nickel diffusion layer 12 and thenickel layer 14. Herewith, the surface-treated steel sheet 1 as shown inFIG. 3 is obtained.

It is to be noted that in the present embodiment, a temper rolling maybe applied to the obtained surface-treated steel sheet 1. Herewith, itis possible to regulate the surface roughness of the surface of thesurface-treated steel sheet 1 to be the inner surface of the batterycontainer; when the surface-treated steel sheet 1 is used as a batterycontainer, the contact area between the battery container and thepositive electrode mixture can be increased, the internal resistance ofthe battery can be decreased, and the battery properties can beimproved.

As described above, the surface-treated steel sheet 1 of the presentembodiment is produced.

In the surface-treated steel sheet 1 of the present embodiment, asdescribed above, by controlling the total amount of the nickel containedin the iron-nickel diffusion layer and the nickel contained in thenickel layer so as to fall within a comparatively small range of 4.4g/m² or more and less than 10.8 g/m², and by setting the thickness ofthe iron-nickel diffusion layer 12 measured with a high frequency glowdischarge optical emission spectrometric analyzer so as to fall within acomparatively thin range of 0.04 to 0.31 μm, with respect to theobtained alkaline battery 2, while the volume percentage is beingremarkably improved by making thin the thickness of the can wall of thebattery container, further it is possible to suppress the iron exposurearea of the steel sheet on the inner surface side of the batterycontainer and to improve corrosion resistance of the battery container.In addition, as described above, by setting the average crystal grainsize in the surface portion of the nickel layer 14 so as to bepreferably 0.2 to 0.6 μm, the hardness of the nickel layer 14 can bemade appropriate, and herewith, the following excellent effect isobtained: when the surface-treated steel sheet 1 is mold-processed intoa battery container, while the exposure of iron due to deep cracksgenerated in the surface-treated steel sheet 1 is being effectivelysuppressed, fine cracks are generated on the inner surface of thebattery container, and consequently the battery properties are moreeffectively improved. Moreover, as described above, by setting thethickness of the nickel layer 14 to be preferably 0.5 μm or more, thecorrosion resistance is more improved when the surface-treated steelsheet 1 is used for the battery container, and it is possible to moreeffectively prevent the gas generation in such an interior of a batteryand the increase of the internal pressure of the interior of the batterydue to the gas generation. Accordingly, the surface-treated steel sheet1 of the present embodiment can be suitably used as the batterycontainers of the batteries such as alkaline batteries, the batteriesusing alkaline electrolytic solutions such as nickel-hydrogen batteries,and lithium-ion batteries.

EXAMPLES

Hereinafter, the present invention is described more specifically withreference to Examples, but the present invention is not limited to theseExamples.

Reference Example A

As a base sheet, there was prepared a steel sheet 11 obtained byannealing a cold rolled sheet (thickness: 0.25 mm) of a low-carbonaluminum-killed steel having the chemical composition shown below:

C: 0.045% by weight, Mn: 0.23% by weight, Si: 0.02% by weight, P: 0.012%by weight, S: 0.009% by weight, Al: 0.063% by weight, N: 0.0036% byweight, the balance: Fe and inevitable impurities.

Then, the prepared steel sheet 11 was subjected to alkaline electrolyticdegreasing and sulfuric acid immersion pickling, then subjected toelectrolytic plating under the below-described conditions, and thus anickel plating layer 13 was formed on the steel sheet 11 so as to have adeposition amount of 10.7 g/m². Subsequently, as for the thickness ofthe nickel plating layer 13, the deposition amount thereof wasdetermined by performing a fluorescent X-ray measurement. The resultsthus obtained are shown in Table 1.

Bath composition: nickel sulfate: 250 g/L, nickel chloride: 45 g/L,boric acid: 45 g/L

pH: 3.5 to 4.5

Bath temperature: 60° C.

Electric current density: 20 A/dm²

Energizing time: 18 seconds

Next, the steel sheet 11 having the nickel plating layer 13 formedthereon was subjected to a thermal diffusion treatment by continuousannealing under the conditions of a heat treatment temperature of 430°C., a heat treatment time of 1 minute, and a reductive atmosphere, andthus an iron-nickel diffusion layer 12 and a nickel layer 14 wereformed, to obtain a surface-treated steel sheet 1.

Next, the obtained surface-treated steel sheet 1 was subjected to atemper rolling under the condition of an extension percentage of 1%.

Then, by using the surface-treated steel sheet 1 after the temperrolling, according to the below-described methods, the measurement ofthe thickness of the iron-nickel diffusion layer 12 and the thickness ofthe nickel layer 14, the measurement of the surface hardness of thenickel layer 14, the measurement of the average crystal grain size ofthe nickel layer 14, and the observation of the surface with a scanningelectron microscope (SEM) were performed.

<Measurement of Thickness of Iron-Nickel Diffusion Layer 12 andThickness of Nickel Layer 14>

With respect to the surface-treated steel sheet 1, by using a highfrequency glow discharge optical emission spectrometric analyzer, thevariations of the Fe intensity and the Ni intensity were continuouslymeasured in the depth direction from the outermost layer toward thesteel sheet 11, the time giving the Fe intensity of 10% of the saturatedvalue of the Fe intensity is taken as the starting point, themeasurement time until the time giving the Ni intensity an intensity of10% of the maximum value of the Ni intensity after the Ni intensity hadexhibited the maximum value thereof was calculated, and on the basis ofthe calculated measurement time, the thickness of the iron-nickeldiffusion layer 12 was determined. It is to be noted that when thethickness of the iron-nickel diffusion layer 12 was determined, thethickness of the iron-nickel diffusion layer 12 was measured, first, onthe basis of the results (FIG. 5(C)) obtained by performing the highfrequency glow discharge optical emission spectrometric analysis of thebelow-described nickel-plated steel sheet (Comparative Example 1)undergoing no thermal diffusion treatment, the measurements wereperformed by taking as the reference thickness the thickness measured asthe iron-nickel diffusion layer (the value obtained by converting themeasurement time into the thickness as follows: in FIG. 5(C), the timegiving the Fe intensity 10% of the saturated value of the Fe intensitywas taken as the starting point, the measurement time until the timegiving the Ni intensity 10% of the maximum value after the Ni intensityhad exhibited the maximum value thereof was converted into thethickness). It is to be noted that the reference thickness was 0.30 μm.In addition, the thickness of the actual iron-nickel diffusion layer 12in Example 1 was determined by subtracting the reference thickness fromthe measurement result of the thickness of the iron-nickel diffusionlayer 12 of the surface-treated steel sheet of Example 1. In addition,for the nickel layer 14, by taking as the starting point the time atwhich the measurement of the surface of the surface-treated steel sheet1 was started with the high frequency glow discharge optical emissionspectrometric analyzer, the measurement time until the Fe intensity wasgiven an intensity of 10% of the saturated value of the Fe intensity wascalculated, and on the basis of the calculated measurement time, thethickness of the nickel layer 14 was determined. Then, on the basis ofthe measurement result, the ratio of the thickness of the iron-nickeldiffusion layer 12 to the thickness of the nickel layer 14 (thickness ofiron-nickel diffusion layer 12/thickness of nickel layer 14) wasdetermined. The results thus obtained are shown in FIG. 5(A) andTable 1. It is to be noted that, in Table 1, the ratio of (thickness ofiron-nickel diffusion layer 12/thickness of nickel layer 14) wasdescribed as “Thickness ratio Fe—Ni/Ni.”

It is to be noted that in relation to the measurement on the highfrequency glow discharge optical emission spectrometric analyzer, thereference thickness calculated from the measurement of the nickelplating layer comes to be thick with the increase of the thickness ofthe nickel plating layer; and accordingly, it is preferable to examinethe reference thickness in each of the plating amount when theiron-nickel diffusion layer is determined, or alternatively, it ispreferable to determine the thickness of the iron-nickel diffusion layerby deriving the relation formula between the plating amount and thereference thickness by performing the reference thickness measurement oftwo or more samples different from each other in the plating amount,before being subjected to heat treatment.

<Measurement of Surface Hardness of Nickel Layer 14>

For the nickel layer 14 of the surface-treated steel sheet 1, thesurface hardness measurement was performed by measuring the Vickershardness (HV) with a micro hardness tester (model: MVK-G2, manufacturedby Akashi Seisakusho Co., Ltd.), by using a diamond indenter, under theconditions of a load of 10 gf and a holding time of 10 seconds. Theresults thus obtained are shown in FIG. 6.

<Measurement of Average Crystal Grain Size of Nickel Layer 14>

First, the surface of the surface-treated steel sheet 1 was etched.Specifically, 0.1 ml of an aqueous solution prepared by dissolvingcopper sulfate hydrate in a concentration of 200 g/L was dropwise placedon the surface of the surface-treated steel sheet 1, immediatelythereafter 0.1 ml of hydrochloric acid was dropwise placed on the samesurface, the aqueous solution and the hydrochloric acid were held for 30seconds to perform etching, and subsequently the surface of thesurface-treated steel sheet 1 was washed with water and dried. Next, abackscattered electron image of the surface of the surface-treated steelsheet 1 was obtained by using a scanning electron microscope (SEM), fourstraight line segments were drawn on the obtained backscattered electronimage as shown in FIG. 7(A), and according to the above-describedmethod, from the number of the crystal grains located on the individualstraight line segments, the average crystal grain size of the nickellayer 14 was calculated. The results thus obtained are shown in FIG.7(A) and Table 1.

<Observation of Surface with Scanning Electron Microscope (SEM)>

By using the surface-treated steel sheet 1, a LR6 (JIS) batterycontainer was prepared in such a way that the nickel layer 14 was placedon the inner surface side of the battery container, and the thickness ofthe can wall is 0.15 mm. Then, in the obtained battery container, theportions of 10 mm, 25 mm, and 40 mm from the bottom were measured with ascanning electron microscope (SEM) and an energy dispersion type X-rayanalysis, and thus the backscattered electron images, the element mapsof iron, and the element maps of nickel were obtained. The results thusobtained are shown in FIGS. 8(A) to 8(C). It is to be noted that inFIGS. 8(A) to 8(C), the images denoted by “Image” are the backscatteredelectron images, and the images denoted by “Feka” are the element mapsof iron and the images denoted by “Nika” are the element maps of nickel.It is to be noted that in the element maps of iron, the portions wherethe kα lines due to iron were observed are white. Also, in the elementmaps of nickel, similarly, the portions where the kα lines due to nickelwere observed are white. The images of the element maps of iron werebinarized with an image processing software, and the area proportion ofthe white portions in relation to the whole of the obtained image(namely, the exposed area proportion of iron) was measured. The resultsthus obtained are shown in FIG. 14 and Table 2.

Example 1

A surface-treated steel sheet 1 was prepared in the same manner as inReference Example A except that the heat treatment temperature was setto be 600° C. when the thermal diffusion treatment was performed for thesteel sheet 11 having a nickel plating layer 13 formed thereon; and themeasurements and the observations were pertained in the same manner. Theresults thus obtained are shown in FIGS. 5(B), 6, 7(B), 9, and 14, andTables 1 and 2.

Comparative Example 1

A nickel-plated steel sheet was prepared under the same conditions as inExample 1 except that the steel sheet 11 having a nickel plating layer13 formed thereon was not subjected to either of the thermal diffusiontreatment and the temper rolling. Then, the prepared nickel-plated steelsheet was subjected to measurements, as described above, on the basis ofthe high frequency glow discharge optical emission spectrometricanalysis to obtain the measurement results shown in FIG. 5(C), and themeasurements were performed by taking as the reference thickness thethickness measured as the iron-nickel diffusion layer (the valueobtained by converting the measurement time into the thickness asfollows: in FIG. 5(C), the time giving the Fe intensity 10% of thesaturated value of the Fe intensity was taken as the starting point, themeasurement time until the time giving the Ni intensity 10% of themaximum value after the Ni intensity had exhibited the maximum valuethereof was converted into the thickness). It is to be noted that inComparative Example 1, the surface hardness and the average crystalgrain size of the nickel plating layer 13 were measured, in place of thesurface hardness and the average crystal grain size of the nickel layer14. It is to be noted that in Comparative Example 1, when the averagecrystal grain size of the nickel plating layer 13 was measured, thesurface etching was not performed. The results thus obtained are shownin FIGS. 5(C), 6, 7(C), 10, and 14, and Tables 1 and 2.

Comparative Example 2

A surface-treated steel sheet 1 was prepared in the same manner as inExample 1 except that the heat treatment temperature was set to be 700°C. when the thermal diffusion treatment was performed for the steelsheet 11 having a nickel plating layer 13 formed thereon, and no temperrolling was performed; and the measurements and the observations wereperformed in the same manner It is to be noted that in ComparativeExample 2, when the average crystal grain size of the nickel layer 14was measured, the surface etching was not pertained. The results thusobtained are shown in FIGS. 5(D), 6, 7(D), 11, and 14, and Tables 1 and2.

TABLE 1 After heat treatment Before heat Heat treatment Iron-nickeltreatment Conditions diffusion Nickel layer 14 Plating amountTemperature Time Nickel layer 14 layer 12 Thickness ratio Averagecrystal (g/m²) [° C.] [min] Thickness [μm] Thickness [μm] Fe—Ni/Ni grainsize [μm] Reference Example A 10.7 430 1 1.12 More than 0 μm, less 0<  0.21 than 0.04 μm Example 1 10.7 600 1 1.17 0.24 0.205 0.45 ComparativeExample 1 10.7 — — Reference thickness Reference thickness — 0.05*Comparative Example 2 10.7 700 1 0.65 0.5  0.769 0.93 *The averagecrystal grain size of the nickel plating layer 13 was measured.

TABLE 2 Heat treatment conditions Temper- Exposed area ature TimeMeasurement proportion [° C.] [min] positions of iron [%] Reference 4301 10 mm from bottom 8.46 Example A 25 mm from bottom 7.12 40 mm frombottom 5.52 Example 1 600 1 10 mm from bottom 10.08 25 mm from bottom9.63 40 mm from bottom 7.84 Comparative — — 10 mm from bottom 6.85Example 1 25 mm from bottom 5.15 40 mm from bottom 6.15 Comparative 7001 10 mm from bottom 12.74 Example 2 25 mm from bottom 17.63 40 mm frombottom 18.50

Reference Example 1

As a base sheet, the same steel sheet 11 as in Example 1 was prepared.Then, the prepared steel sheet 11 was subjected to alkaline electrolyticdegreasing and sulfuric acid immersion pickling, then subjected toelectrolytic plating under the below-described conditions, and thus anickel plating layer 13 was formed on the steel sheet 11 so as to have athickness of 20 μm. It is to be noted that as for the thickness of thenickel plating layer 13, the deposition amount thereof was determined byperforming a fluorescent X-ray measurement.

Bath composition: Nickel sulfate: 250 g/L, nickel chloride: 45 g/L,boric acid: 45 g/L

pH: 3.5 to 4.5

Bath temperature: 60° C.

Then, a backscattered electron image of the surface of the steel sheet11 having the nickel plating layer 13 formed thereon was obtained bymeasuring with a scanning electron microscope (SEM). The results thusobtained are shown in FIGS. 12(A) and 13(A). It is to be noted that FIG.13(A) is an image obtained by enlarging FIG. 12(A).

Next, for the steel sheet 11 having the nickel plating layer 13 formedthereon, the measurement of the surface hardness of the nickel platinglayer 13 was performed by the same method as in the above-describedmeasurement of the surface hardness of the nickel layer 14. The resultsthus obtained are shown in FIG. 15.

Reference Example 2

A steel sheet 11 was prepared in the same manner as in Reference Example1, and a nickel plating layer 13 was formed on the steel sheet 11. Next,the steel sheet 11 having the nickel plating layer 13 formed thereon wassubjected to a thermal diffusion treatment by continuous annealing underthe conditions of a heat treatment temperature of 300° C., a heattreatment time of 41 seconds, and a reductive atmosphere, and thus aniron-nickel diffusion layer 12 and a nickel layer 14 were formed, toobtain a surface-treated steel sheet 1.

Then, a backscattered electron image of the surface of the obtainedsurface-treated steel sheet 1 was obtained in the same manner as inReference Example 1, and the measurement of the surface hardness of thenickel layer 14 was performed. The results thus obtained are shown inFIG. 15.

Reference Examples 3 to 8

Surface-treated steel sheets 1 were prepared in the same manner as inReference Example 2 except that when the steel sheet 11 having a nickelplating layer 13 formed thereon was subjected to the thermal diffusiontreatment, the heat treatment temperature was set to be 400° C.(Reference Example 3), 500° C. (Reference Example 4), 600° C. (ReferenceExample 5), 700° C. (Reference Example 6), 800° C. (Reference Example7), and 900° C. (Reference Example 8); the measurements were performedin the same manner as in Reference Example 1. The results thus obtainedare shown in FIGS. 12(B) to 12(E), 13(B) to 13(E), and 15.

As shown in Table 1, in Reference Example A and Example 1 in each ofwhich the thickness of the iron-nickel diffusion layer 12 is 0.04 to0.31 μm, and the total amount of the nickel contained in the iron-nickeldiffusion layer and the nickel contained in the nickel layer is 4.4 g/m²or more and less than 10.8 g/m², the kα lines of iron originating fromthe diffusion of the steel sheet 11 up to the surface of thesurface-treated steel sheet 1 were not observed as shown in the elementmaps of iron in FIGS. 8 and 9. Specifically, as shown in FIG. 14 andTable 2, in each of Reference Example A and Example 1, the areaproportion (the exposed area proportion of iron) of the white portionsin relation to the whole of the element map of iron was as small as 11%or less, and the kα lines of iron originating from the diffusion of thesteel sheet 11 up to the surface of the surface-treated steel sheet 1were not observed. It is to be noted that in the element map of iron ineach of FIGS. 8 and 9, the white portions in which the kα line of ironwas observed were sparsely present, and this was probably caused by theextremely slight exposure of the steel sheet 11 through fine scratcheson the surface of the surface-treated steel sheet 1, and it can bedetermined that such white portions were not caused by the diffusion ofthe steel sheet 11 up to the surface of the surface-treated steel sheet1, due to the thermal diffusion treatment.

In addition, as shown in FIG. 7, in Reference Example A and Example 1involving the application of the thermal diffusion treatment, ascompared with Comparative Example 1 free from the application of thethermal diffusion treatment, it can be seen that with the increase ofthe heat treatment temperature in the thermal diffusion treatment, thecrystal grains grew (namely, the crystal grain sizes were increased).This is probably ascribable to the progress of the recrystallization ofthe grains constituting the nickel layer 14 to increase the crystalgrain sizes with the increase of the heat treatment temperature.

In addition, as shown in FIG. 15, from the measurement results of thesurface hardness in Reference Examples 1 to 8, it can be verified thatby setting the heat treatment temperature in the thermal diffusiontreatment to be higher than 300° C., the surface hardness of the nickellayer 14 was decreased (the nickel plating layer 13 is softened). Thisfact suggests that the recrystallization of the grains constituting thenickel layer 14 was allowed to progress. It is to be noted that inReference Examples 1 to 8, the thickness of the nickel plating layer 13was set to be as thick as 20 μm, and accordingly, the hardness of thenickel layer 14 was able to be appropriately measured, without beingaffected by the steel sheet 11 serving as the substrate. Accordingly, itis conceivable that in Reference Example A and Example 1 setting theheat treatment temperature of the thermal diffusion treatment to be 400°C. or 630° C., the recrystallization of the grains constituting thenickel layer 14 is allowed to progress, and the surface hardness of thenickel layer 14 is allowed to be appropriate.

On the other hand, as shown in Table 1, Comparative Example 1 free fromthe application of the thermal diffusion treatment, the average crystalgrain size of the nickel layer 14 ended up with less than 0.2 μm.Herewith, it is conceivable that in Comparative Example 1, the surfacehardness of the nickel layer 14 came to be too high, and when thesurface-treated steel sheet 1 was mold-processed into a batterycontainer, deep cracks to reach the steel sheet 11 were caused in thesurface-treated steel sheet 1, the iron of the steel sheet 11 wasexposed, and the corrosion resistance of the battery container wasdegraded.

In addition, as shown in Table 1, in Comparative Example 2 setting theheat treatment temperature of the thermal diffusion treatment to be 700°C., the thickness of the iron-nickel diffusion layer 12 ended up to be0.5 μm or more, and the thickness of the nickel layer 14 came to be lessthan 0.85 μm; herewith, as shown in the element maps of iron in FIG. 11,the kα lines of iron originating from the diffusion of the iron of thesteel sheet 11 up to the vicinity of the surface of the surface-treatedsteel sheet 1 were observed. In other words, as compared withabove-described FIGS. 8 and 9, in the element maps of iron in FIG. 11,the white points originating from the kα lines of iron are larger innumber, and moreover present in a getting together manner (actually,when referring to FIG. 14 and Table 2, in the element maps of iron ineach of FIGS. 8 and 9 (Reference Example A, Example 1), the areaproportions (the exposed area proportion of iron) of the white portionsin relation to the whole of the image is 11% or less; on the other hand,in the element maps of iron of FIG. 11 (Comparative Example 2), theproportions concerned are as high as 15% or more at the positions of 25mm and 40 mm from the bottom).

In particular, in Comparative Example 2 adopting a heat treatmenttemperature of 700° C., as compared with Reference Example A and Example1 adopting heat treatment temperatures of 430° C. and 600° C.,respectively, the exposed area proportion of iron is increased at any ofthe positions of 10 mm, 25 mm and 40 mm, respectively, from the bottomof the battery container. Such an increase of the exposed areaproportion of iron in Comparative Example 2 is particularly remarkableat the positions of 25 mm and 40 mm from the bottom of the batterycontainer. This indicates that when the battery container is formed byusing the surface-treated steel sheet 1, with the increase of thedistance from the bottom of the battery container, namely, with thedecrease of the distance from the opening of the battery container, thesurface-treated steel sheet 1 undergoes the drawing forces in thestretching direction (the press direction during being molded into thebattery container) and the circumferential direction, and consequentlyundergoes a higher load processing in each of these directions.

It is conceivable that in Comparative Example 2 adopting a heattreatment temperature of 700° C., the thermal diffusion treatmentproceeded to an excessive extent, to make the thickness of the nickellayer 14 too thin, and accordingly, in the portions undergoing theapplication of the above-described high load processing (the portionsdistant from the bottom of the battery container), the exposure of theiron of the steel sheet 11 is increased. This can also be verified fromthe element maps of iron and the element maps of nickel, shown in FIG.11. Specifically, when referring to FIG. 11, in the element maps ofiron, with the increase of the distance from the bottom of the batterycontainer, the exposed area proportion of iron is increased, and theportions allowing iron to be exposed in a getting together manner areincreased. When such element maps of iron are compared with thecorresponding element maps of nickel, the positions of the portionsallowing iron to be exposed in a getting together manner in the elementmaps of iron correspond to the positions of the portions having smallerdetection amounts of nickel in the element maps of nickel. Herewith, itis possible to verify that in Comparative Example 2, the exposure ofiron is increased in the thinner-thickness portions of the nickel layer14.

From this fact, it is conceivable that the diffusion of the iron of thesteel sheet 11 up to the vicinity of the surface of the surface-treatedsteel sheet 1 allows the iron-nickel diffusion to proceed to anexcessive extent, consequently the nickel layer of the surface layercomes to be too thin, and accordingly, when the surface-treated steelsheet 1 is press molded, the coating with nickel on the inner surface ofthe battery can comes to be incomplete. Moreover, as shown in Table 1,in Comparative Example 2, the average crystal grain size of the nickellayer 14 was more than 0.6 μm, and as can be seen from FIG. 6, theVickers hardness was lower as compared with the Vickers hardnesses ofReference Example A and Example 1.

It is to be noted that as far as the numerical values of the exposedarea proportion of iron shown in FIG. 14 and Table 2, ComparativeExample 1 free from the application of the thermal diffusion treatmenttends to exhibit lower values as compared with Reference Example A andExample 1 undergoing a thermal diffusion treatment at 430° C. and 600°C., respectively. However, when referring to the element maps shown inFIG. 10, in Comparative Example 1, as compared with the element maps ofReference Example A and Example 1 shown in FIGS. 8 and 9, respectively,the exposure of iron tends to occur at specific positions in a gettingtogether manner, and in particular, this tendency is remarkable in theelement map at a position of 10 mm from the bottom of the batterycontainer. This is probably ascribable to the too hard nickel platinglayer 13 on the surface in Comparative Example 1 free from theapplication of a thermal diffusion treatment. In other words, when thesurface-treated steel sheet 1 is molded into a battery container, thebottom portion of the battery container is brought into contact with thecircumferential portion of the punch used for press to undergo bendingwork; the surface-treated steel sheet 1 of Comparative Example 1 has thetoo hard nickel plating layer 13 on the surface, and accordingly, duringthis bending work, deep cracks tend to be generated on the surface ofthe surface-treated steel sheet 1. Accordingly, in the case where thesurface-treated steel sheet 1 of Comparative Example 1 is used as abattery container, when deep cracks are generated on the surfacethereof, iron is locally exposed to be dissolved into the electrolyticsolution, and gas is liable to be generated in the interior of thebattery along with the dissolution of iron. The generation of such gasis liable to increase the internal pressure in the interior of thebattery.

Example 2

As a base sheet, there was prepared a steel sheet 11 obtained byannealing a cold rolled sheet (thickness: 0.25 mm) of a low-carbonaluminum-killed steel having the chemical composition shown below:

C: 0.045% by weight, Mn: 0.23% by weight, Si: 0.02% by weight, P: 0.012%by weight, S: 0.009% by weight, Al: 0.063% by weight, N: 0.0036% byweight, the balance: Fe and inevitable impurities

Then, the prepared steel sheet 11 was subjected to alkaline electrolyticdegreasing and sulfuric acid immersion pickling, then subjected toelectrolytic plating under the below-described conditions, and thus anickel plating layer 13 was formed on the steel sheet 11 so as to have adeposition amount of 8.9 g/m². Subsequently, as for the thickness of thenickel plating layer 13, the deposition amount thereof was determined byperforming a fluorescent X-ray measurement.

Bath composition: Nickel sulfate: 250 g/L, nickel chloride: 45 g/L,boric acid: 45 g/L

pH: 3.5 to 4.5

Bath temperature: 60° C.

Electric current density: 20 A/dm²

Energizing time: 16 seconds

Next, the steel sheet 11 having the nickel plating layer 13 formedthereon was subjected to a thermal diffusion treatment by continuousannealing under the conditions of a heat treatment temperature of 480°C., a heat treatment time of 30 seconds, and a reductive atmosphere, andthus an iron-nickel diffusion layer 12 and a nickel layer 14 wereformed, to obtain a surface-treated steel sheet 1.

Next, the obtained surface-treated steel sheet 1 was subjected to atemper rolling under the condition of an extension percentage of 1%. Thethickness of the surface-treated steel sheet 1 after the temper rollingwas 0.250 mm.

Then, by using the surface-treated steel sheet 1, according to theabove-described method, the thicknesses of the iron-nickel diffusionlayer 12 and the nickel layer 14 were measured. In addition, on thebasis of the measured results, the ratio of the thickness of theiron-nickel diffusion layer 12 to the thickness of the nickel layer 14(thickness of iron-nickel diffusion layer 12/thickness of nickel layer14) was determined. The results thus obtained are shown in Table 3.

Examples 3 to 9

In each of Examples 3 to 9, a surface-treated steel sheet 1 was obtainedin the same manner as in Example 3 except that the thickness of thenickel plating layer 13, and the continuous annealing conditions (heattreatment conditions) for the steel sheet 11 having a nickel platinglayer 13 formed thereon were altered as shown in Table 3, and themeasurements were performed in the same manner. The results thusobtained are shown in Table 3.

Comparative Example 3

A nickel-plated steel sheet was prepared under the same conditions as inExample 3 except that neither a continuous annealing nor a temperrolling was performed after the formation of the nickel plating layer13. The results thus obtained are shown in Table 3.

Comparative Examples 4 to 6

In each of Comparative Examples 4 to 6, a surface-treated steel sheet 1was obtained in the same manner as in Example 3 except that thethickness of the nickel plating layer 13, and the continuous annealingconditions (heat treatment conditions) for the steel sheet 11 having anickel plating layer 13 foiled thereon were altered as shown in Table 3,and the measurements were performed in the same manner. The results thusobtained are shown in Table 3.

TABLE 3 After heat treatment Before heat Heat treatment Iron-nickeltreatment conditions diffusion Plating amount Temperature Nickel layer14 layer 12 Thickness ratio (g/m²) [° C.] Time Thickness [μm] Thickness[μm] Fe—Ni/Ni Example 2 10.1 480 30 sec 1.09 0.10 0.092 Example 3 4.5500 30 sec 0.47 0.07 0.159 Example 4 9.2 500 30 sec 0.97 0.13 0.13 Example 5 9.9 500 30 sec 1.04 0.14 0.135 Example 6 10.0 500 60 sec 1.050.14 0.133 Example 7 10.1 550 30 sec 1.06 0.15 0.142 Example 8 9.8 58060 sec 1.00 0.20 0.200 Example 9 10.7 600 30 sec 1.07 0.26 0.243Comparative 9.0 — — 1.00 Absent — Example 3 Comparative 9.7 500 60 min0.91 0.35 0.39  Example 4 Comparative 9.9 700 30 sec 0.89 0.44 0.494Example 5 Comparative 18.6 700 30 sec 1.87 0.45 0.241 Example 6

Next, the surface-treated steel sheets 1 of Examples 3 and 4 andComparative Examples 4 to 6, and the nickel-plated steel sheet ofComparative Example 3 were evaluated according to the below-describedmethod, with respect to the corrosion resistance when each of thesesteel sheets was molded into a battery container.

<Evaluation of Corrosion Resistance (1)>

A blank was prepared by punching out a surface-treated steel sheet 1into a predetermined shape with a press machine, the obtained blank wassubjected to a drawing processing under the below-described conditionsin such a way that the nickel layer 14 was on the inner surface side,and thus a battery container was prepared (it is to be noted that when anickel-plated steel sheet was used, a battery container was prepared insuch a way that the nickel plating layer 13 was on the inner surfaceside). Specifically, a tubular body was obtained by applying a drawingand ironing processing to the blank by using a drawing and ironingmachine including drawing dies (or ironing dies) arranged in six stagesand a punch, and a battery container was obtained by cutting the lugpart in the vicinity of the opening of the obtained tubular body. Thedrawing processing used the dies in each of which the clearance was setin such a way that the thickness of the can wall at a position of 10 mmfrom the can bottom after processing was ±5%.

Next, the obtained battery container was evaluated with respect to theamount of Fe ions dissolved as follows: the obtained battery containerwas filled with a 10 mol/L potassium hydroxide solution, sealed andstored under the conditions of 60° C., 480 hours, then the amount of Feions dissolved from the inner surface of the battery container into thesolution was measured with a high frequency inductively coupled plasmaemission spectrometric analyzer (ICP) (ICPE-9000, manufactured byShimadzu Corp.), and the amount of Fe ions dissolved was evaluated onthe basis of the following standards. When the evaluation was A or B inthe following standards, the dissolution of iron from the inner surfaceof the battery container was determined to be sufficiently suppressed.The results thus obtained are shown in Table 4.

A: The amount of Fe ions dissolved was less than 33 mg/L.

B: The amount of Fe ions dissolved was 33 to 35 mg/L.

C: The amount of Fe ions dissolved was more than 35 mg/L.

TABLE 4 Before heat After heat treatment treatment Heat treatmentIron-nickel Plating conditions diffusion Thickness Evaluation ofcorrosion amount Temperature Nickel layer 14 layer 12 ratio resistance(1) (g/m²) [° C.] Time Thickness [μm] Thickness [μm] Fe—Ni/NiEvaluation*¹ Example 3 4.5 500 30 sec 0.47 0.07 0.159 A Example 4 9.2500 30 sec 0.97 0.13 0.13 A Comparative 9.0 — — 1.0 Absent — B Example 3Comparative 9.7 500 60 min 0.91 0.35 0.39 B Example 4 Comparative 9.9700 30 sec 0.89 0.44 0.494 C Example 5 Comparative 18.6 700 30 sec 1.870.45 0.241 B Example 6 *¹Evaluation standards: A: less than 33 B: 33 to35 C: more than 35

As shown in Table 4, Examples 3 and 4 in each of which the thickness ofthe iron-nickel diffusion layer 12 was 0.04 to 0.31 μm, and the totalamount of the nickel contained in the iron-nickel diffusion layer andthe nickel contained in the nickel layer was 4.4 g/m² or more and lessthan 10.8 g/m² gave the results that Examples 3 and 4 were bothevaluated as excellent in corrosion resistance. In other words, it hasbeen verified that Examples 3 and 4 each have a corrosion resistanceequal to or higher than the corrosion resistances of ComparativeExamples 5 and 6, when Comparative Examples 5 and 6 having the corrosionresistances equal to or higher than the corrosion resistances ofconventional surface-treated steel sheets are taken as references.

On the other hand, as shown in Table 4, Comparative Example 3 free fromthe application of a thermal diffusion treatment was good in theevaluation result of the corrosion resistance, but did not form theiron-nickel diffusion layer 12 due to the omission of the thermaldiffusion treatment, and is accordingly conceived to be poor in theadhesiveness of the nickel plating layer 13.

Moreover, even in the case where a thermal diffusion treatment wasperformed, the thickness of the iron-nickel diffusion layer 12 was madetoo thick due to an excessive thermal diffusion treatment, iron wasprobably exposed to the surface of the nickel layer 14; as in the caseof Comparative Example 5, when Comparative Example 6 having thecorrosion resistance equivalent to the corrosion resistances ofconventional surface-treated steel sheets was taken as the reference,Comparative Example 5 gave the result that Comparative Example 5 wasequal to or lower than this Comparative Example 6 with respect to thecorrosion resistance.

In addition, it is conceivable that the surface-treated steel sheet ofComparative Example 6 was too large in the total amount of the nickelcontained in the iron-nickel diffusion layer and the nickel contained inthe nickel layer was too large (the thickness of the nickel platinglayer 13 was too thick), and accordingly gave a too thick can wall anddecreased the volume percentage when used as a battery container.

Next, the surface-treated steel sheet 1 of each of Examples 3 and 4 andComparative Example 4 was subjected to a drawing and ironing processingwith a higher load than in the corrosion evaluation (1) to prepare abattery container, according to the below-described method, andevaluated with respect to the corrosion resistance of the batterycontainer, under more severe conditions.

<Evaluation of Corrosion Resistance (2)>

The preparation of a battery container and the measurement of the amountof Fe ions dissolved were performed in the same manner as in theevaluation of corrosion resistance (1) except that a battery containerwas prepared by performing a drawing processing with a higher load thanin the evaluation of corrosion resistance (1) with respect to the sixstages of drawing dies (or ironing dies) in the drawing and ironingmachine, as described below, and the evaluation was performed on thebasis of the following standards. The results thus obtained are shown inTable 5.

The drawing and ironing processing used the dies in each of which theclearance was set in such a way that the thickness of the can wall at aposition of 10 mm from the can bottom after processing was 0.15 mm.

In addition, the amount of Fe ions dissolved from the inner surface ofthe battery container into the solution was evaluated on the basis ofthe following standards. When the evaluation was A or B in the followingstandards, the dissolution of iron from the inner surface of the batterycontainer was determined to be sufficiently suppressed.

A: The amount of Fe ions dissolved was less than 35 mg/L.

B: The amount of Fe ions dissolved was 35 to 38 mg/L.

C: The amount of Fe ions dissolved was more than 38 mg/L.

TABLE 5 Before heat After heat treatment treatment Heat treatmentIron-nickel Plating conditions diffusion Thickness Evaluation ofcorrosion amount Temperature Nickel layer 14 layer 12 ratio resistance(2) (g/m²) [° C.] Time Thickness [μm] Thickness [μm] Fe—Ni/NiEvaluation*² Example 3 4.5 500 30 sec 0.47 0.07 0.159 B Example 4 9.2500 30 sec 0.97 0.13 0.13 A Comparative 9.1 500 60 min 0.91 0.35 0.39 CExample 4 *²Evaluation standards: A: less than 35 B: 35 to 38 C: morethan 38

As shown in Table 5, Examples 3 and 4 in each of which the thickness ofthe iron-nickel diffusion layer 12 was 0.04 to 0.31 μm, and the totalamount of the nickel contained in the iron-nickel diffusion layer andthe nickel contained in the nickel layer was 4.4 g/m² or more and lessthan 10.8 g/m² gave the results that Examples 3 and 4 were evaluated asexcellent in corrosion resistance even in the case where the batterycontainers were prepared by performing the drawing and ironingprocessing with a higher load than in the above-described evaluation ofcorrosion resistance (1)

On the other hand, as shown in Table 5, in Comparative Example 4 havinga too thick thickness of the iron-nickel diffusion layer 12 due to anexcessive thermal diffusion treatment, it is conceivable that the ironwas exposed to the surface of the nickel layer 14, and when a batterycontainer was prepared by performing a drawing and ironing processingwith a high load, Comparative Example 4 gave a result that the corrosionresistance was poor.

REFERENCE SIGNS LIST

-   1 . . . surface-treated steel sheet    -   11 . . . steel sheet    -   12 . . . iron-nickel diffusion layer    -   13 . . . nickel plating layer    -   14 . . . nickel layer-   2 . . . alkaline battery    -   21 . . . positive electrode can        -   211 . . . positive electrode terminal    -   22 . . . negative electrode terminal    -   23 . . . positive electrode mixture    -   24 . . . negative electrode mixture    -   25 . . . separator    -   26 . . . current collector    -   27 . . . gasket    -   28 . . . insulating ring    -   29 . . . exterior case

1. A surface-treated steel sheet for a battery container, comprising: asteel sheet, an iron-nickel diffusion layer formed on a first surface ofthe steel sheet to be an inner surface of the battery container; a firstnickel layer formed on the iron-nickel diffusion layer and constitutingthe outermost layer; and a second nickel layer constituting an outermostlayer on a second surface of the steel sheet to be an outer surface ofthe battery container, wherein when the Fe intensity and the Niintensity are continuously measured from the first surface of thesurface-treated steel sheet for a battery container along the depthdirection with a high frequency glow discharge optical emissionspectrometric analyzer, the thickness of the iron-nickel diffusion layerbeing the difference (D2−D1) between the depth (D1) at which the Feintensity exhibits a first predetermined value and the depth (D2) atwhich the Ni intensity exhibits a second predetermined value is 0.04 to0.31 μm; and the total amount of the nickel contained in the iron-nickeldiffusion layer and the nickel contained in the first nickel layer is4.4 g/m² or more and less than 10.8 g/m², wherein the depth (D1)exhibiting the first predetermined value is the depth exhibiting anintensity of 10% of the saturated value of the Fe intensity measured bythe above-described measurement, and the depth (D2) exhibiting thesecond predetermined value is the depth exhibiting an intensity of 10%of the maximum value when the measurement is further performed along thedepth direction after the Ni intensity shows the maximum value by theabove-described measurement.
 2. The surface-treated steel sheet for abattery container according to claim 1, wherein the average crystalgrain size in the surface portion of the first nickel layer is 0.2 to0.6 μm.
 3. The surface-treated steel sheet for a battery containeraccording to claim 1, wherein the thickness of the first nickel layer is0.4 to 1.2 μm.
 4. The surface-treated steel sheet for a batterycontainer according to claim 1, wherein the Vickers hardness (HV) of thefirst nickel layer measured with a load of 10 gf is 200 to
 280. 5. Thesurface-treated steel sheet for a battery container according to claim1, wherein the thickness of the iron-nickel diffusion layer is 0.05 to2.7 μm.
 6. A battery container made of the surface-treated steel sheetfor a battery container according to claim
 1. 7. A battery provided withthe battery container according to claim
 6. 8. A method for producing asurface-treated steel sheet for a battery container, comprising: forminga first nickel plating layer on a first surface of a steel sheet to bean inner surface of the battery container with a nickel amount of 4.4g/m² or more and less than 10.8 g/m²; forming a second nickel platinglayer on a second surface of the steel sheet to be an outer surface ofthe battery can; and applying a heat treatment to the steel sheet havingthe first nickel plating layer and the second nickel plating layerformed thereon by maintaining the steel sheet at a temperature of 450 to600° C. for 30 seconds to 2 minutes to thereby form an iron-nickeldiffusion layer having a thickness of 0.04 to 0.31 μm and a first nickellayer constituting an outermost layer of the first surface.